General physiology: fluid compartment shifts: practice question

A Physiology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Physiology · General physiology: fluid compartment shifts

Sample item

A healthy adult is given 1 litre of 0.9% saline intravenously and allowed to equilibrate. Which change is expected?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Extracellular volume rises, intracellular volume is unchanged and osmolarity is unchanged

Normal saline is isotonic with plasma, so there is no osmotic gradient across cell membranes and no net water movement into or out of cells. Sodium is confined largely to the extracellular space, so the whole litre stays there, about a quarter in plasma and three quarters in interstitial fluid.

Option by option

  • A. Extracellular volume rises, intracellular volume is unchanged and osmolarity is unchanged (correct)

    Isotonic fluid expands only the extracellular compartment without changing osmolarity.

  • B. Extracellular volume rises, intracellular volume falls and osmolarity rises

    Intracellular shrinkage with a rise in osmolarity is the effect of hypertonic saline.

  • C. Both compartments expand in proportion to their size and osmolarity falls

    Proportional expansion with falling osmolarity would follow a hypotonic fluid such as 5% dextrose after glucose is metabolised.

  • D. Extracellular volume is unchanged, intracellular volume rises as water follows sodium

    Isotonic saline does not drive water into cells; sodium stays extracellular.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Physiology question

What is the answer to this Physiology question?

The best answer is A, Extracellular volume rises, intracellular volume is unchanged and osmolarity is unchanged. Normal saline is isotonic with plasma, so there is no osmotic gradient across cell membranes and no net water movement into or out of cells.

Why is "Extracellular volume rises, intracellular volume falls and osmolarity…" not the answer?

Intracellular shrinkage with a rise in osmolarity is the effect of hypertonic saline.

Why is "Both compartments expand in proportion to their size and osmolarity f…" not the answer?

Proportional expansion with falling osmolarity would follow a hypotonic fluid such as 5% dextrose after glucose is metabolised.

Why is "Extracellular volume is unchanged, intracellular volume rises as wate…" not the answer?

Isotonic saline does not drive water into cells; sodium stays extracellular.

What concept does this question test?

It tests General physiology: fluid compartment shifts in Physiology. Sodium is confined largely to the extracellular space, so the whole litre stays there, about a quarter in plasma and three quarters in interstitial fluid.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study General physiology: fluid compartment shifts?

For each system, sketch the key graph (pressure–volume loop, oxygen dissociation curve, nephron handling) and explain it aloud. Then use the flashcards and the other Physiology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.